This invention relates to photographically produced stencils for use in screen printing (herein, for the sake of brevity, referred to as photostencils).
In screen printing the image to be printed is defined by masking off whole or part apertures of a screen printing mesh with stencil material. Thus, during printing, this stencil material prevents the passage of screen printing ink through the masked areas.
Although such stencils may be produced manually it is normal practice to produce them photographically. The production of such a photostencil requires the preparation of a dry water-soluble photosensitive polymer layer which, on imagewise exposure to suitable illumination, is made water-insoluble in the exposed areas such that the layer material may be washed away with cold or warm water in the unexposed areas to produce a photostencil. Such photosensitive layers used in screen printing have in practice been generally based on polyvinyl alcohol, polyvinyl alcohol--polyvinyl acetate mixtures, or gelatin, sensitised by a water-soluble dichromate salt, a diazonium salt or a ferric salt.
Two different methods have been used for applying the photostencil to the screen mesh; these are termed the direct and indirect methods. In the direct method, an aqueous solution of polyvinyl alcohol, usually mixed with polyvinyl acetate aqueous emulsion and containing the sensitiser, to give about 10-30% by volume of nonvolatile material, is coated as a viscous liquid onto the tensioned screen mesh and dried by evaporation. Since substantial contraction of the layer thickness occurs on drying, several coats are generally applied, each with intermediate drying, to build up sufficient dry layer thickness to provide a photostencil with adequate mechanical strength. The dry sensitized mesh is then exposed by contact to a high contrast photographic positive in a vacuum frame to obtain close contact with the positive. The exposing radiation may be mercury metal halide, mercury vapour, pulsed xenon or carbon arc radiation, all of which have high intensity in the range 350-450 nm to which the sensitised polymer is most sensitive. The image is developed by carefully washing out the unexposed polymer with cold or warm water, and the remaining polymer is then dried again to form the photostencil.
In the indirect method, an aqueous solution of sensitised polymer, usually at 10-25% concentration by volume, is coated as a thick layer onto a colourless transparent film base, such as polyester film, which may be coated to facilitate subsequent dry stripping. After a long drying stage in a complex drying tunnel, the dry polymer layer is exposed to a high contrast photographic positive in a vacuum frame using the same illumination as in the direct method. The image is developed usually by washing out the soluble, unexposed polymer with cold or warm water and, while wet, the remaining polymer, still on the film base, is applied under light pressure to the screen mesh and dried in situ. Finally the film base is stripped off to leave the printing stencil on the mesh.
The direct and indirect methods for producing photostencils both therefore require at least two lengthy drying stages under carefully controlled conditions to avoid drying defects and dust contamination. Also the final stencil is not resistant to water-based screen inks nor to many inks containing organic solvents with hydroxyl groups. Water-based screen inks are widely used in textile printing on both flat and cylindrical screens and then the stencil must be hardened chemically or protected by baking on a protective lacquer.
Moreover, in the direct method, the evaporation of water during the first drying stage may cause shrinkage and loss of image quality; particularly the formation of saw-tooth edges due to defective bridging of mesh apertures. Multiple coats with intermediate drying are generally used to reduce this effect, but limited print detail resolution is still obtained in the direct method. The indirect method also suffers from some print quality loss due to shrinkage but the main problem is one of poor adhesion since the stencil is applied to the mesh in a swollen but not liquid condition and thus does not encapsulate the mesh. Both polyvinyl alcohol and gelatin have poor specific adhesion to polyester monofilament mesh which is the most widely used mesh in screen printing.